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The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells or T lymphocytes mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. Histologically, each lobe of the thymus can be divided into a central medulla and a peripheral cortex which is surrounded by an outer capsule. The cortex and medulla play different roles in the development of T-cells. Cells in the thymus can be divided into thymic stromal cells and cells of hematopoietic origin (derived from bone marrow resident hematopoietic stem cells). Developing T-cells are referred to as thymocytes and are of hematopoietic origin. Stromal cells include epithelial cells of the thymic cortex and medulla, and dendritic cells. The thymus provides an inductive environment for development of T cells from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance. The thymus is largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced by adipose (fat) tissue. Nevertheless, residual T lymphopoiesis continues throughout adult life. ==Structure== The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces. At birth it is about 5 cm in length, 4 cm in breadth, and about 6 mm in thickness. The organ enlarges during childhood, and atrophies at puberty. Unlike the liver, kidney and heart, for instance, the thymus is at its largest in children. The thymus reaches maximum weight (20 to 37 grams) by the time of puberty. The thymus of older people is scarcely distinguishable from surrounding fatty tissue. As one ages the thymus slowly shrinks, eventually degenerating into tiny islands of fatty tissue. By the age of 75 years, the thymus weighs only 6 grams. In children the thymus is grayish-pink in colour and in adults it is yellow. If examined when its growth is most active, the thymus will be found to consist of two lateral lobes placed in close contact along the middle line, situated partly in the thorax, partly in the neck, and extending from the fourth costal cartilage upward, as high as the lower border of the thyroid gland. It is covered by the sternum, and by the origins of the sternohyoid and sternothyroid muscles.〔 Below, it rests upon the pericardium, being separated from the aortic arch and great vessels by a layer of fascia. In the neck, it lies on the front and sides of the trachea, behind the sternohyoidei and sternothyreoidei. The two lobes differ slightly in size and may be united or separated.〔 Each lateral lobe is composed of numerous lobules held together by delicate areolar tissue; the entire organ being enclosed in an investing capsule of a similar but denser structure. The primary lobules vary in size from that of a pin's head to that of a small pea, and are made up of a number of small nodules or follicles. The follicles are irregular in shape and are more or less fused together, especially toward the interior of the organ. Each follicle is from 1 to 2 mm in diameter and consists of a medullary and a cortical portion, and these differ in many essential particulars from each other. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thymus」の詳細全文を読む スポンサード リンク
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